Capacitive touchpad is an input device for controlling cursor movement by providing smooth panel for finger to slide on it. Since capacitive touchpad is very thin, it can thus be designed into ultra-thin notebook, key board, digital player or other devices, besides, its non-mechanical design makes it very easy to be maintained.
FIG. 1 is the prior art two dimensional capacitive touchpad 100, comprising panel 102, Y axis sensing layer 104, insulated layer 106, X axis sensing layer 108 and bottom board 110. When finger 112 touches panel 102, the sensed value (capacitance value) on the touched location will have a change, and the control circuit connected to touchpad 100 can convert the capacitance on the touchpad into sensed value, as shown in FIG. 2, in order to judge finger location, amount of movement and direction of movement. In the prior art technology, the detected sensed value is used to judge if the object touches capacitive touchpad 100, this is as shown in FIG. 3, when sensed value is larger than critical value th, it means object touches touchpad 100, on the contrary, when sensed value is smaller than critical value th, it means object leaves touchpad 100 or object is not on touchpad 100.
However, touchpad 100 might be interfered by signals from wireless device such as cellular phone, etc., and generate noise, therefore, touchpad 100 might misjudge that object is touching, clicking, moving or performing any gesture operation on it. FIG. 4A is an example showing noise generated on touchpad 100, the noise is converted into sensed value through analog digital converter, as shown in FIG. 4B, finally through a sampling process, the waveform as in FIG. 4C is obtained, this kind of waveform is similar to the waveform generated when two fingers touch touchpad 100, as shown in FIG. 5, therefore, noise in FIG. 4A could be misjudged to be two fingers touching touchpad 100, this could lead to wrong operation.
Therefore, a detection method which can prevent interference from noise is highly expected.